1. Field of the Invention
The present invention relates to a power supply device having a function of compensating for an instantaneous voltage drop in an alternating current power supply voltage, and specifically, relates to technology for increasing the efficiency and reducing the size of the device.
2. Description of the Background Art
FIG. 3 shows first heretofore known technology of a power supply device having a function of compensating for an instantaneous voltage drop in an alternating current power supply voltage.
In FIG. 3, reference sign 1 is an alternating current power supply, and a rectifier circuit formed of diodes 2 to 5 is connected to both ends of the alternating current power supply. An inductor 6 and a semiconductor switching device 7, such as a MOSFET, are connected in series to either end of a series circuit of the diodes 4 and 5, and a diode 8 and smoothing capacitor 9 are connected in series to either end of the switching device 7.
Direct current input terminals of an inverter INV formed of semiconductor switching devices 10 to 13 are connected to either end of the smoothing capacitor 9, and a primary winding 14a of a transformer 14 is connected to alternating current output terminals of the inverter INV.
A rectifier circuit formed of diodes 15 to 18 is connected to both ends of a secondary winding 14b of the transformer 14. An inductor 19 and a capacitor 20 are connected in series to either end of a series circuit of the diodes 17 and 18, and a load 21 is connected to both ends of the capacitor 20.
Functions required of the above-described power supply device are as follows:
converting an alternating current input voltage to a direct current voltage of a desired size, and keeping output voltage constant regardless of fluctuation in input voltage and load current;
isolating an alternating current input unit and a direct current output unit; and
controlling an alternating input current to a sinusoidal wave with a power factor of practically 1.
Furthermore, when the load 21 is one of which reliability is required, as with an information and communication instrument, the power supply device is required to have a function whereby it is possible to maintain a constant output voltage even in the event of a drop in the voltage of the alternating current power supply 1 for a period of in the region of a few milliseconds to a few cycles, a so-called instantaneous voltage drop (this function will hereafter be referred to as an instantaneous voltage drop compensation function).
A description will be given, while referring to FIGS. 4A to 4E, of operations for realizing these functions.
In FIG. 4A, an input voltage Vin from the alternating current power supply 1 has a sinusoidal waveform, while the output voltage (rectified voltage) of the rectifier circuit formed of the diodes 2 to 5 has the waveform indicated by Vr1 in FIG. 4B. Herein, on the switching device 7 of FIG. 3 being turned on when Vin has, for example, positive polarity, current flows along a path from the alternating current power supply 1 through the diode 2, inductor 6, switching device 7, and diode 5 to the alternating current power supply 1, the rectified voltage Vr1 is applied across the inductor 6, and a current IL increases.
Also, on the switching device 7 being turned off, the current IL flows along a path from the alternating current power supply 1 through the diode 2, inductor 6, diode 8, smoothing capacitor 9, and diode 5 to the alternating current power supply 1. At this time, a voltage equivalent to the difference between a voltage Ed1 of the smoothing capacitor 9 and the input voltage Vin is applied to the inductor 6, but as Ed1 is kept higher than a peak value of Vin by an operation of the circuit, the current IL decreases.
It is possible to control the waveform and amplitude of the current IL by controlling the duty ratio of the turning on and off of the switching device 7. When the waveform of the current IL is the kind of sinusoidal rectified waveform in FIG. 4B (ripple is ignored for the sake of simplification), an input current Iin has a sinusoidal waveform, as shown in FIG. 4A. Also, by controlling the amplitude of IL in accordance with the load power, it is possible to keep the voltage Ed1 of the smoothing capacitor 9 constant, and thus possible to keep the output voltage of the inverter INV constant.
Herein, FIGS. 4C to 4E show operations and voltage waveforms of each portion of the switching devices 10 to 13 when there is and is not an instantaneous voltage drop compensation function.
The inverter INV formed of the switching devices 10 to 13 converts the voltage Ed1 of the smoothing capacitor 9 into a high frequency alternating current voltage. A positive voltage Vt is applied to the primary winding 14a of the transformer 14 on the switching devices 10 and 13 being turned on, while a negative voltage Vt is applied to the primary winding 14a on the switching devices 11 and 12 being turned on, as shown in FIGS. 4C and 4D. By the positive and negative voltages Vt being applied alternately to the primary winding 14a in this way, the high frequency alternating current voltage Vt is input into the transformer 14 (the cycles of Vin and Vt are represented as being of the same extent in FIGS. 4A to 4E for the sake of easier understanding, but in general, Vin is of a commercial frequency of 50 or 60 Hz, while Vt is of a frequency of a few kilohertz or more in order to reduce the size of the transformer).
The transformer 14 isolates and transforms the input high frequency alternating current voltage Vt and, after converting the voltage across the secondary winding 14b of the transformer 14 into a rectified voltage Vr2 of FIG. 4E using the rectifier circuit formed of the diodes 15 to 18, smoothes the voltage using the inductor 19 and capacitor 20, and applies the voltage to the load 21 as an output voltage Vout. The output voltage Vout can be controlled by the duty ratio (hereafter referred to as the inverter duty ratio) of the turning on of the switching devices 10 and 13 or switching devices 11 and 12.
Operations when the power supply device includes an instantaneous voltage drop compensation function and the alternating current power supply 1 is sound are as shown under “Normal time” in FIGS. 4C to 4E.
As opposed to this, under “Time of instantaneous voltage drop”, the input power decreases due to the occurrence of an instantaneous voltage drop in the voltage of the alternating current power supply 1, the voltage Ed1 drops, and the amplitude of the voltage Vt also decreases. However, provided that the drop in the voltage Ed1 is within a predetermined range, it is possible to keep the average value of the voltage Vt constant by increasing the duty ratio of the turning on of the switching devices 10 and 13 or switching devices 11 and 12, and to maintain the predetermined rectified voltage Vr2 and, by extension, the output voltage Vout.
However, when an instantaneous voltage drop compensation function is provided, the efficiency of the device decreases. This is for the following reasons.
In order to maintain the constant output voltage Vout even when the voltage Ed1 has dropped to a certain extent, it is necessary that the transformation ratio of the transformer 14 (the value of n in n:1, which is the turn ratio between the primary winding 14a and secondary winding 14b) is smaller than an essentially known optimum value. For example, when the voltage Ed1 is kept constant at 400V and the output voltage Vout is 10V at a normal time, the transformation ratio of the transformer 14 necessary to operate the inverter INV at a maximum duty ratio is 400:10, that is, n=40 (for the sake of simplification, voltage drop in the circuit is ignored here).
Meanwhile, the transformation ratio necessary in order to maintain the output voltage Vout at 10V even when the voltage Ed1 drops as far as 200V is 200:10, that is n=20. When setting the transformation ratio n under this condition, operation is carried out with an inverter duty ratio of approximately 0.5 in order to maintain Vout at 10V at a normal time, when the voltage Ed1 is 400V.
In this case, the amplitude of current flowing on the primary side of the transformer 14 is 1/n that of current flowing through the inductor 19, but when the power supply device has an instantaneous voltage drop compensation function, the transformation ratio n, whose original optimum value is 40, becomes 20, and the value of current flowing on the primary side of the transformer 14 increases due to the providing of the instantaneous voltage drop compensation function. Because of this, loss occurring in the switching devices 10 to 13 and primary winding 14a increases.
The rectified voltage Vr2 is approximately Ed1/n, but as the voltage applied to the diodes 15 to 18 at a normal time increases when the transformation ratio n decreases, it is necessary to use parts with a high breakdown voltage as the diodes 15 to 18. Generally, there is a tendency for loss in a semiconductor part to increase under the same conditions the higher the breakdown voltage, because of which loss occurring in the device increases.
Also, at a normal time and when there is an instantaneous voltage drop compensation function, a period for which the voltage Vr2 is not applied lengthens, because of which the value of the inductance of the inductor 19 necessary in order to smooth the voltage Vr2 increases. In the case in which there is no instantaneous voltage drop compensation function in FIGS. 4D and 4E, the rectified voltage Vr2 is not applied for the very short time in which the polarity of the voltage Vt switches in the period during which the rectified voltage Vr2 drops from a predetermined value to 0V, but at a normal time and when there is an instantaneous voltage drop compensation function, the rectified voltage Vr2 is not applied for a period equivalent to one-half of one cycle, and for this period it is necessary that energy is supplied to the load 21 by the inductor 19.
For these reasons, the inductor 19 increases in size, which results in an increase in the overall size of the device and an increase in loss occurring in the inductor 19.
The circuit shown in FIG. 5 is known as second heretofore known technology whereby the above-described kind of increase in loss from the inverter INV onward is avoided.
In FIG. 5, an inductor 22 and a diode 24 are connected in series to a positive side direct current bus between the smoothing capacitor 9 and inverter INV, and a semiconductor switching device 23 is connected between a connection point of the inductor 22 and diode 24 and a negative side direct current bus. Also, a smoothing capacitor 25 is connected between the cathode of the diode 24 and the negative side direct current bus. A step-up chopper is configured of the inductor 22, diode 24, switching device 23, and smoothing capacitor 25. As the other configurations in FIG. 5 are the same as in FIG. 3, a description thereof will be omitted.
It is possible to control current flowing through the inductor 22 with the step-up chopper, using the same kind of operation as that of the circuit formed of the inductor 6, switching device 7, and diode 8, as a result of which it is possible to obtain an output voltage Ed2 higher than the input voltage Ed1. That is, using an operation of the step-up chopper, it is possible to keep the voltage Ed2 of the capacitor 25 constant (for example, 400V) even when the voltage Ed1 of the capacitor 9 drops (for example, from 400V to 200V) when there is an instantaneous voltage drop in voltage, and a design wherein the transformation ratio is n=40 is possible in the previously described example.
A circuit wherein a drop in alternating current power supply voltage is compensated for using the above-described step-up chopper is shown in JP-A-2-241371 (page 2, bottom right column, line 1 to page 3, top left column, line 4, FIG. 2 and the like).
However, the circuit shown in FIG. 5 has the following drawbacks.
That is, the inductor 22, switching device 23, and diode 24 configuring the step-up chopper generate loss. While inevitable in a case in which the switching device 23 is carrying out a turning on or turning off operation, loss also occurs when the voltage Ed1 is sufficiently high and the switching device 23 is stopped, due to the winding resistance of the inductor 22 and the forward voltage drop of the diode 24. Because of this loss, the amount by which the circuit loss from the inverter INV onward is reduced is cancelled out.
Also, as current is constantly flowing through the inductor 22, an inductor 22 with a large current capacity is needed, despite the time for which the stepping-up operation is carried out being extremely short.
Furthermore, a large capacity smoothing capacitor 25 is needed in order to absorb ripple current generated from the inverter INV. As the one smoothing capacitor 9 has a sufficiently large capacity in order to supply energy when there is an instantaneous voltage drop, the smoothing capacitor 9 can perform the role of absorbing both the ripple current generated by the circuit formed of the inductor 6, switching device 7, and diode 8 and the ripple current of the inverter INV in the circuit of FIG. 3.
However, as the inductor 22 is inserted between the inverter INV and smoothing capacitor 9 in the circuit of FIG. 5, a high frequency ripple current can not pass. Because of this, the separate smoothing capacitor 25 is needed, because of which the size of the device increases.
A circuit wherein energy when there is an instantaneous voltage drop is supplied by another capacitor charged in advance is shown in JP-A-8-185993 (paragraphs [0023] to [0028], FIG. 1 and the like). However, as the circuit disclosed in JP-A-8-185993 (paragraphs [0023] to [0028], FIG. 1 and the like) is also such that a capacitor that absorbs ripple at a normal time and a capacitor that supplies energy when there is an instantaneous voltage drop are separated, it is not possible to avoid an increase in the size of the device.
The circuit shown in FIG. 6 is known as third heretofore known technology whereby the above-described increase in the size of the device is avoided.
In FIG. 6, reference sign 101 is a bypass diode connected between the cathodes of the diodes 8 and 24, while the other configurations are the same as in FIG. 5. At a normal time when the alternating current power supply 1 is sound, the switching device 23 does not operate, and the step-up chopper formed of the inductor 22, switching device 23, diode 24, and capacitor 25 is bypassed by the diode 101. Herein, as the voltages Ed1 and Ed2 are smoothed direct current voltages both at a normal time and when there is a instantaneous voltage drop, the diode 101, unlike the diode 24, does not need to be capable of rectifying a high frequency, so it is possible to use a low speed diode. As the forward voltage of a low speed diode is in the region of one-half compared to that of a high speed diode, and no current flows through the inductor 22 at a normal time, it is possible to reduce loss considerably in comparison with that in the circuit of FIG. 5.
Furthermore, as the time for which the inductor 22 is energized is a few tens of milliseconds or less when there is an instantaneous voltage drop, it is possible to use a short time rated inductor with thin windings as the inductor 22, and thus possible to reduce the size of the inductor 22 considerably in comparison with that in the circuit of FIG. 5. Also, as there is no longer an inductor interposed between the smoothing capacitor 9 and inverter INV owing to the bypassing operation of the diode 101, it is possible for the smoothing capacitor 9 to absorb the ripple current of the inverter INV too at a normal time. Because of this, it is sufficient that the smoothing capacitor 25 withstands only the ripple of the step-up chopper and inverter INV during an instantaneous voltage drop period, and thus possible to use a capacitor with an extremely small capacity.
This method is shown in, for example, JP-A-2010-41910 (paragraphs [0040] to [0046], FIG. 1 and the like).